DIGITAL VIDEO
WHAT IS IT?
The Telecommunications Act of 1996
This act was passed by congress and signed by the president. One of its provisions requires all terrestrial TV stations in the
country to convert to digital modulation. (Contrary to a persistent rumor, the VHF channels will not be abandoned.)
The deadline for this switch is February 17, 2009. To stay competitive, all cable systems are rapidly converting to digital, but there
is no deadline for that.
The pre-existing TV technology is called analog. It is also called NTSC (National Television System Committee), which are the
people who defined it. The NTSC spec was created in 1946, updated for color in 1953, and updated for stereo in 1984. Both of
these updates were backward compatible, rendering nobody’s TV set obsolete. But the new digital standard is totally different.
The only thing it has in common with NTSC is the 6 mega-hertz channel width. To continue using an NTSC TV after 2009 you
might have to buy a converter box, which will probably cost about $60. These boxes are not yet available. You will not need such
a box if you can rely on a cable or satellite box that has an NTSC output.
The new digital standard is called ATSC (Advanced Television Standards Committee). The government will require that all new
TVs be able to receive ATSC channels. The ATSC standard includes multiple formats from 640x480 pixels to 1920x1080 pixels.
All TVs must receive all of these digital formats and display them suitably. The broadcaster chooses the format.
To make the transition gradual, the FCC is temporarily giving all terrestrial TV stations a second channel, so that they can
broadcast a digital channel along with their analog channel until 2009. There are 1700 terrestrial TV stations in the U.S. 1550 of
them have their digital channel on the air. Most affiliates of the seven major networks (about 850) transmit some high definition
programs. Over 95% of the U.S. population can receive some high-def programming from these stations. (These numbers are as
of December 2005.)
The bad news:
The cost to consumers of the new hardware.
Home TV systems may be especially complicated during the transition.
The picture-in-picture feature many people enjoy will largely disappear as TV set designers concentrate on more important things,
but it will eventually make a comeback.
NTSC images sometimes look worse on a big high-def set than on a small standard TV.
The good news:
The quality of TV reception will improve dramatically. Temporary inconvenience, permanent improvement.
What exactly is ATSC?
“Advanced Television System Committee” is the name of the technical standard that defines the
digital TV (DTV) that the FCC has chosen for terrestrial TV stations.
ATSC employs MPEG-2, a data compression standard. MPEG-2 typically achieves a 50-to-1
reduction in data.
It achieves this by not retransmitting areas of the screen that have not changed since
the previous frame.
Digital cable TV systems and DBS systems like DirecTV have devised their own standards
that differ somewhat from ATSC. Their high-def set top boxes (STBs) conform to ATSC
at their output connectors. Most of these systems use MPEG-2.
(DirecTV and Dish Network are delivering systems that use MPEG-4.)
Bandwidth
The bandwidth for NTSC is always 6 MHz.
Without data compression, the bandwidth for 1080i would be 300 MHz.
With MPEG-2 data compression the bandwidth varies according to how fast the image changes.
For 480i the bandwidth rarely goes above 1 MHz. For 1080i and 720p the bandwidth rarely goes above 3 MHz.
Thus it is possible to put six 480i programs or two 1080i programs in a 6 MHz channel.
The FCC allows this. Thus terrestrial DTV stations have sub-channels.
It is up to the station managers how many sub-channels to have and what programming will air on those sub-channels.
Note that a sub-channel showing a static image (e.g. a weather map or bulletin board)
requires almost no bandwidth despite being at high resolution.
ATSC is an imperfect standard in that occasionally the bandwidth requirement will exceed the channel size.
When this happens, the picture can get blurry or jumpy. Jumpiness occurs when frames are deleted.
Blurriness is preferred because it is only momentary and often not noticed.
Transmission encoders have improved gradually and hopefully will continue to do so.
In the future perhaps they will fail in a completely unnoticeable manner.
Which is better: 1080i or 720p?
1080i and 720p require about the same bandwidth when showing live action:
A 1080i image has twice as many pixels, while 720p shows twice as many frames per second.
While showing films at 24 frames per second, 720p requires about half the bandwidth of 1080i.
A common opinion is that 720p is better for sporting events,
while 1080i looks better for documentaries, dramas, and most things that come 24 frames per second.
Unfortunately the networks are picking one format for all their shows. ABC, ESPN, and FOX have chosen 720p.
All other networks are using 1080i.
Hopefully some day they will choose the format according to the content.
1080i and 720p are called High Definition TV (HDTV).
480p is called Enhanced Definition TV (EDTV).
480i is Standard Definition TV (SDTV).
TV channels are assigned a 6 MHz slot.
EXAMPLE OF AN ANALOG NTSC CHANNEL
The bottom edge of the over-the-air channel 2
is 54 MHz and the upper edge is 60 MHz.
Within this 6 MHz space is a video carrier, a color carrier, and an audio carrier.
The frequency of the video carrier is 1.25 MHz above the lower edge,
so for channel 2 the video is at 55.25 MHz.
The color carrier is approx. 3.58 MHz above the video carrier
so for ch 2 it is 58.83 MHz.
The audio carrier is 4.5 MHz above the video carrier, so for ch 2 it is 59.75 MHz.
ANALOG vs. DIGITAL
Spectral display
CURRENT CHANNEL ASSIGNMENTS
2 55.25 59.75
3 61.25 65.75
4 67.25 71.75
5 77.25 81.75
60 * 747.25 751.75
6 83.25 87.75
61 * 753.25 757.75
62 * 759.25 763.75
Channels 2 – 6 to be reallocated!!
Channels 60-69 will become the 700 MHz Band
63 * 765.25 769.75
64 * 771.25 775.75
65 * 777.25 781.75
66 * 783.25 787.75
67 * 789.25 793.75
68 * 795.25 799.75
69 * 801.25 805.75
8VSB HUH?
8VSB is the Transmission method for HDTV and means 8-level vestigial sideband modulation
Like traditional NTSC, the 8-VSB format
utilizes a vestigial sideband approach in the interest
of conserving spectrum space.
Unlike NTSC, however, 8-VSB takes this concept to greater extremes:
the lower RF sideband is almost completely removed.
Hum!! Lets see, SSB carrier inserted..
SOUND FAMILIAR?
HD Radio???
It has been reported that as of June 2008, over 1,700 AM and FM
stations are broadcasting with HD Radio technology.
Most of the stations that have adopted the technology are FMs,
while AM stations have been slower to adopt the brand.
Normal FM
FM and Digital
Sending pure digital data through the narrow 10 kilohertz AM channel is
roughly equivalent to sending
data through an analog telephone line (~33.6k), thus limiting the maximum
throughput possible. By using Spectral Band Replication the HDC+SBR codec
is able to recreate sounds equal to or exceeding 15,000 Hz, thus achieving
FM quality on the bandwidth-tight AM band. The HD Radio AM hybrid mode
offers two options which can carry approximately 40 or 60 kbit/s of data,
but most AM-digital stations default to the more-robust 40 kbit/s mode which features
redundancy (same signal broadcast twice).
Currently, FM stations in the United States and Canada
are licensed to carry 130 kilohertz of audio modulation bandwidth,
requiring approximately 260 kilohertz of RF spectrum.
Only 15 kHz of the modulation bandwidth is used by analog Monaural audio.
Analog stereo uses 53 kHz of space, and RBDS is centered at 57 kHz.
The "remainder" is currently available for other services, including rental
for secondary broadcast services, paging and datacasting, radio reading service,
or as a transmitter-studio link for in-house telemetry.
As with AM, FM stations may use separate exciters to modulate the very different signals.
A combiner is often used, either before common amplification or after separate
amplification, though stations are also now allowed to use a separate
transmitting antenna slightly higher or lower on the radio tower.
In each case the ratio of power of the analog signal to the digital signal is
standardized at 100:1. The 1% power level of the digital signal from FM stations
is sufficient to approximate the coverage area of the analog signal. Going forward,
advances in digital tuner design will create the scenario where the digital signal
coverage will probably exceed the analog coverage due to the greater potential
ability to extract digital information from a noisy signal.
QUESTIONS???